Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is demonstrated in surface capacitive systems as illustrated in FIG. 1. Surface capacitive systems, for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981) and US 201010259503A1 (Yanase, Oct. 14, 2010), usually employ a single non-conductive substrate (01) which has a conductive layer (02) on its underside. A small AC voltage (03) is imposed upon the conductive layer at several points of the substrate, resulting in a uniform electrostatic field. When a conductor, such as a human finger (04), touches the uncoated surface, a capacitor (05) is dynamically formed enabling the flow of current to and from the user's finger. The sensor's controller can determine the location of the touch input event indirectly from the change in the capacitance as measured from the corners of the panel by examining the relative magnitudes of the induced currents (06) sourced at each corner. Although simple, surface capacitive type systems are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another method of capacitive sensing applied to touch panels can be found in projected capacitive systems. As can be seen in FIG. 2, the projected capacitance system comprises a drive electrode (10) and sense electrode (12) which are formed on a substrate (not shown). The drive electrode is fed with a changing voltage or excitation signal (11) which induces a signal on the sense electrode (12) by means of a coupling capacitor (13) formed between the electrodes. When a conductor such as a human finger (04) is brought to close proximity to both electrodes, it forms a first dynamic capacitor (14) with the drive electrode and a second dynamic capacitor (15) with the sense electrode. The effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the two electrodes effectively modulating the induced signal (16) at the sense electrode (12). This concept has been widely applied to touch panel systems where a plurality of projected capacitive sensors is employed allowing multiple simultaneous touch input events to be detected.
The projected capacitive method overcomes the problem of detecting multiple simultaneous touch input events associated with surface capacitance type systems by employing an array of electrodes arranged in horizontal rows and vertical columns wherein the array is sequentially driven by an excitation signal and sequentially scanned to generate capacitance measurement signals. FIG. 3 shows such an array as well as a schematic block diagram of a well-known type of projected capacitive system as disclosed in, for example, U.S. Pat. No. 7,663,607 (Hotelling, Feb. 16, 2010). In this particular case, each of the vertical columns is formed by a drive electrode (20) and each of the horizontal rows by a sense electrode (21). A drive circuit (22) under the control of a host controller (23) and a multiplexer (30) is used to sequentially apply an excitation signal (24) to each drive electrode in turn, such that, only a single drive electrode receives the excitation signal during one scan period. During this scan period, each sense electrode (21) is connected to sense circuit (25) by means of sample and hold (S&H) circuitry and a multiplexer (26) wherein the sense circuit is used to measure the signal generated by means of capacitance coupling between the currently active drive electrode (20) and the selected sense electrode (21). By means of scanning the location of the drive and sense electrode, a plurality of capacitance measurements which correspond to each row and column intersection or sensing point (27) may be generated. These measurements are then digitized by an analog-to-digital converter (28), further processed by a signal processing unit (29) to extract the touch co-ordinates and finally sent to the host controller (23). However, in spite of the multi-touch capabilities of the projected capacitive method, it has some significant limitations. For example, it cannot be used to detect the force of touch input and is unable to detect touch input from non-conductive objects such as a plastic stylus or pen.
In order to overcome these limitations, hybrid systems incorporating force sensing devices into projected capacitive touch panels have been proposed. “Metal-polymer composite with nanostructured filler particles and amplified physical properties”, Applied Physics Letters 88, 102013 (2006), discusses a force sensitive material called Quantum Tunneling Composite (QTC) which may be used to form a ring around the periphery of the touch panel. The peripheral ring of QTC material provides a measure of the force being applied to the touch panel whilst the projected capacitive sensor detects the location of touch input. Alternatively, U.S. Pat. No. 6,492,979 (Kent, Dec. 10, 2002) describes a touch panel system incorporating discrete force sensing devices. As shown in FIG. 4, this system includes a touch panel (33) and a display (34) wherein the touch panel comprises a projected capacitive sensor (31) which provides measurement of the touch input location (35) and four strain sensors (32), positioned at the four corners of the display, which provide a measurement of the force of touch input. A processor (36) controls the touch panel and the display and determines the coordinates of the touch. The force sensors (32) provide a measurement of the force of the touch input event with the aid of a monitor (38) and a discriminator (37) to verify the validity of the touch and its coordinates.
As another alternative, the force sensor may be formed in the touch panel sensing layer itself. For example, U.S. Pat. No. 5,915,285 (Sommer, Jun. 22, 1999) (FIG. 5) describes a planar strain gauge structure in which the strain gauge (41) is formed on a transparent substrate (44) by a transparent material such as Indium Tin Oxide (ITO). A first terminal (42) and a second terminal (43) are provided at either end of the structure. The geometry of the gauge is chosen to be sensitive to tension and compression forces along one particular axis by appropriate design of the sensor electrodes. Accordingly, strain along that axis causes a change in resistance between the first terminal (42) and the second terminal (43). US2010/0128002 (Stacy, May 27, 2010) further describes the incorporation of such a transparent strain gauge into a projected capacitive touch panel device. FIG. 6 illustrates this arrangement whereby the strain gauge electrodes (51) are inter-digitated with but electrically isolated from the touch sensor electrodes (52).
A significant limitation of all of the hybrid touch panel devices described above however, is that even though the projected capacitive sensor is capable of determining the location of multiple simultaneous touch input events, the force sensor is incapable of uniquely determining the input force associated with each individual touch input event. It is therefore impossible to calculate, for example, the force being applied by any one particular input object and the utility of the device is greatly limited.